1. Technical Field
The present invention relates to a laminated hard coating film. More particularly, the present invention relates to a laminated hard coating film with high hardness and excellent properties.
This application claims the benefit of Korean Patent Application No. 10-2012-0092530, filed on Aug. 23, 2012, and Korean Patent Application No. 10-2013-0089105, filed on Jul. 26, 2013, which are all hereby incorporated by reference in their entireties into this application.
2. Description of the Related Art
With the advance of mobile appliances such as smart phones, tablet PCs and the like, substrates for displays have recently been required to become lighter and slimmer. Display windows or front panels of such mobile appliances are generally made of glass or reinforced glass both of which have excellent mechanical properties. However, glass suffers from the disadvantage of being heavy and being easily broken by an external impact.
As an alternative to glass, plastic resin films have emerged. Their light weight and resistance to impact are consistent with the trend of pursuing lighter and slimmer mobile appliances. Particularly, a film with high hardness and wear resistance is required. In this regard, it is proposed to utilize a structure in which the substrate is coated with a hard coating layer.
First of all, increasing the thickness of the hard coating layer is considered as an approach to improving the surface hardness thereof. In fact, the hard coating layer should be of a minimal thickness to ensure the surface hardness of the hard coating layer. As the hard coating layer increases in thickness, the surface hardness thereof may become higher. However, a thicker hard coating layer, although increasing the surface hardness, is more prone to setting shrinkage which leads to wrinkling or curling with the concomitant production of cracks or exfoliations, and thus thick hard coating layers are difficult to employ in practice.
Recently, some methods have been proposed for conferring a high hardness on hard coating films, without the problems of cracking and setting shrinkage-induced curling.
Korean Patent Application Publication No. 2010-0041992 discloses a hard coating film composition, free of monomers, comprising a binder resin based on ultraviolet-curable polyurethane acrylate oligomers. However, this hard coating film has a pencil hardness of about 3H, and thus the strength thereof is not sufficient to be a substitute for glass panels for displays.